Pytheas (b c350 BCE) was a Greek explorer and geographer. He was also the first person to write about Britain. Strabo later stated that Pytheas had traveled “the whole of Britain that was accessible.”     https://www.livescience.com/first-western-description-british-isles   Short video about Pytheas by Professor Cunliffe:   Great video on the ancient explorers. (Pytheas begins at 8:50.)